CARACAS, Feb 17 (Reuters) - On a sunny day in the normally placid Altamira Square of affluent east Caracas, several thousand students mill around, debating strategy, painting banners and chanting anti-government slogans.

Suddenly, as dusk falls, a cry goes up "To the highway!", prompting about 200 hardline demonstrators to surge towards a nearby six-lane highway that they intend to block.

Police lines stand in their way but masked youths appear from the shadows to shower them with stones despite shouts of "Stop the violence!" by students behind.

Security forces respond with tear gas and water cannons, and several hours of street battles ensue.

That has been a pattern in Caracas and some other Venezuelan cities in recent days as student protests multiply to demand President Nicolas Maduro's "exit" from power.

The protests began two weeks ago and escalated during rival marches in Caracas last week when three people were shot dead and scores injured in the worst unrest since Maduro won a disputed vote to succeed Hugo Chavez after his death from cancer.

With half an eye to mass movements in Ukraine, the Middle East and Brazil, though with far smaller numbers, the students say they represent a nationwide clamor for change and are the only ones brave enough to stand up to a dictatorial government.

They denounce crime, shortages of food staples and the socialist government's alleged repression of opponents.

"Venezuela has the biggest oil reserves in the world, but you have to queue three hours just to get some milk. What sort of country is that?" said Michael Paredes, 26, an engineering student in the square.

"This government has failed us, and we will stay here until there is change. The rest of Venezuela is asleep. But not us."

Though their numbers have crept up from the dozens to the hundreds, and now a few thousand in some rallies, there is no sign of Venezuelans joining them en masse.

'RICH KIDS'?

After back-to-back presidential election defeats against Chavez in late 2012 and then Maduro, his hand-picked successor, in April 2013, many in the opposition are pessimistic about the chances for quick change.

They feel disenchanted by the failure of massive street demonstrations against Chavez more than a decade ago, after which he expanded his political and economic sway, with measures including nationalizations and strict foreign exchange controls.

With the next presidential election not due until 2019, student leaders hope the protests will gather enough strength to force Maduro's resignation.

The government depicts them as unrepresentative rich kids misled by right-wing politicians and infiltrated by criminals.

"This is a group of privileged Venezuelans, mommy's and daddy's boys, who simply can't stand the idea they are an electoral minority," said Roy Chaderton, Venezuela's envoy to the Organization of American States.

The students are now out daily in Caracas and other flashpoints in western Venezuela like Merida and Tachira states. Reuters reporters have repeatedly seen peaceful demonstrations turn violent as a minority of protesters smash property, hurl rocks, block roads and light fires in the streets.

"Let's not call them students. They are fascist bands," Maduro said, urging his own supporters to flood the streets with cultural events to drown out opponents.

The students are incensed by police tactics, including the arrest of some 100 people, about a dozen of whom remain in jail. Rights groups say some have alleged torture.

But many demonstrators are also angry at radicals in their ranks, some of whom openly admit to violence.

"This government is armed to the teeth, and has all the power. Are we supposed to sit here nicely with our arms folded?" reasoned one 18-year-old protester, gasping for breath in a dark alley of Altamira as he collected more stones for his sling.

"If we want change, we have to confront them, it's the only way," said the student, who asked not to be named.

The majority of students, though, say they are ordinary Venezuelans simply showing a courage their elders lack.

"My father's a barman and has no money to pay for my studies, so I work to fund myself," said Joseph Sandoval, 24, who pays 6,000 bolivars ($950) in fees per semester.

"You can't call me a daddy's boy. Nor him, nor her, nor most of us," he said, pointing at friends at a march.

'WHERE ARE THE MOTHERS?'

While some of the protesters are clearly not wealthy, it is hard to avoid the class overtones to politics in Venezuela where the socialist Chavez surged to power in 1999 amid widespread disgust with the ruling elite's neglect of a poor majority.

Certainly, the opposition demonstrations do appear a more middle-class, lighter-skinned affair than "Chavista" rallies. Students are sometimes keen to show off their English to foreign reporters, or try to outdo each other with clever slogans.

"You need therapy to live in Venezuela," quipped one psychology department's banner. "Maduro: you are the cavity of Venezuela" was the dental students' version.

Some just go along for the fun, or to flirt. Others, like students the world over, are in search of a cause. Cameras are everywhere, with Twitter and Instagram abuzz from the rallies.

It was hardline opposition leaders like Leopoldo Lopez who urged the students onto the streets in the first place, and he now faces charges of murder and terrorism.

But student organizers have seized the initiative.

Some older Venezuelans are joining the marches, although they are a minority, and the protests are a far cry from the huge demonstrations that provided the backdrop to a brief military coup against Chavez in 2002.

"I'm here for my daughter. But where are all the other mothers and fathers? We cannot stay at home," said Belkis Gamboa, 47, protesting alongside her daughter. (Editing by Brian Ellsworth, Kieran Murray and W Simon)